Monitors and printers (hard copiers) and the like are examples of devices that output color data. Monitors express color by additive mixing of color stimuli and hard copiers express color by negative mixing of color stimuli.
In general, a monitor has a color reproduction range that is wider than that of a hard copier and therefore an image displayed by a monitor cannot be reproduced faithfully by a hard copier.
Color-space compression methods are available as a technique for achieving visual matching of colors. These methods include the following:
(1) a method of linearly mapping the entire range of color reproduction;
(2) a method of compressing only color that is outside the color reproduction range to the fringe of the color reproduction range; and
(3) a method of maintaining tonality by compressing color that is outside the color reproduction range to the high-saturation portion of the color reproduction range without performing compression, to the greatest extent possible, within the color reproduction range in order to maintain the original colors.
With method (1), mapping is performed linearly and therefore tonality is not sacrificed. However, this method can give rise to colors that appear different visually.
In accordance with method (2), two points outside the color reproduction range are mapped to the same positions on the fringe of the color reproduction range. Tonality deteriorates and information possessed by the image is lost.
With method (3), tonality is not lost and the original color is preserved in relation to portions of low saturation. Accordingly, this method provides a color image that is most natural to human vision and is used in outputting photographs and the like.
With method (3), however, up to what point within a color reproduction range a color outside the color reproduction range is to be compressed is not defined and it is particularly difficult to set the compressing unit in terms of saturation.
Accordingly, heretofore attention has been directed toward the fact that the method adopted is to perform the conversion in such a manner that the hue of a color signal before color conversion and the hue of a color signal after color conversion will not differ, and therefore the conventional approach is to perform a non-linear mapping taking only a single hue into consideration. With this method, however, the balance between mutually adjacent hues cannot be changed and the hue of a color signal in a first color space cannot be converted linearly to the hue of a color signal in a second color space having a narrower color-space volume.
Further, when a color whose color-reproduction area is greater than that of a monitor is input, compression cannot be performed in optimum fashion based upon a method of color-space compression from the monitor to a hard copier.